If they mate successfully while she is in heat the odd are pretty much 99% she will be pregnant as a result. If there was a cage separator in the way the whole time she was in heat he is unlikely to have been able to mate with her successfully but it is not impossible.
He won't die from not being able to see the girls. If he does get depressed from being on his own the main symptom of this will be he will stop eating and then he will start losing weight. As long as he is still eating he will be ok. If he starts chewing on the bars then I would move him to a different room, if he doesn't chew on the bars I would leave him in the same room as them.
If the girls were to get pregnant the odds are fairly high they will experience some kind of birthing complication, and taking them to a vet may not necessarily save them. There are some things that can go wrong that pretty much guarantee maternal death. For example if they start haemorrhaging post birth there is very little that can be done in time to save them. Things with low odds are saving them is if a pup were to be too big and get stuck the only way to get them out would be a c section, and in guinea pigs that procedure has a very low success rate. However something like a prolapse, or retained placenta, or ongoing moderate bleeding can be fixed by a vet if it is tended to promptly enough and that could save her life.
Taking them prior to a complication happening doesn't really tell you much, the best a vet can do prior to them giving birth is an x-ray once the bones have formed to tell you how many she is expecting. That might tell you whether to expect a pup may get stuck as generally the smaller the litter size the larger the babies will be, so a single baby in an older sow is much more likely to have problems. Aside from that there is no way to tell which guinea pigs are at a higher risk, or what complications to expect until they happen.
